Refactor burndown chart BE logic to support burndown charts in boards

• To get ready for https://gitlab.com/gitlab-org/gitlab-ee/issues/6864, we should refactor the burndown chart BE logic.
• In particular, this issue will not make any user-facing changes. Burndown charts on the milestone page will still behave the same.
• But after this issue, it will make it easier to implement https://gitlab.com/gitlab-org/gitlab-ee/issues/6864.

TODO

• Pass a list of issues, a start date and a end date into Burndown constructor instead of a milestone object.

• Make sure that Gitlab::Seeder::Burndown still works.